In order for a pilot to effectively control an aircraft in all regimes of flight, he must be able to vary the forces acting on it. This is accomplished by deflecting various flaps on the flying surfaces of the aircraft which produce changes in the pressure forces acting on those surfaces. The configuration of an aircraft and its mission determine the shape, size, and placement of the flaps.
While certain types of flaps are used to provide rolling, pitching, and yawing moments which allow changes in the flight path, other types are used to enhance the low speed lift and drag which determine the landing performance of the aircraft. In common use are flaps which are hinged about the trailing edge of the wing which, when deflected downward, increase the camber or a combination of camber and wing area resulting in increased lift and drag. Similarly, there are flaps which pivot about the leading edge of the wing and increase the camber when deflected downward. Both of these devices allow an aircraft to decelerate quickly, approach an airport at a steep descent angle, and land slowly.